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The Sky Chapter 2

The previous chapter took you on a cosmic zoom through space and time. That quick preview only sets the stage for the drama to come. Now it is time to return to Earth and look closely at the sky and answer four essential questions: How do astronomers refer to stars and compare their brightness? How does the sky appear to move as Earth rotates? What causes the seasons? How can astronomical cycles affect Earth’s climate? As you study the sky and its motions, you will be learning to think of Earth as a planet rotating on its axis. The next chapter will introduce you to some of the most dramatic cycles in the sky. Guidepost

I. The Stars A. Constellations B. The Names of the Stars C. Favorite Stars D. The Brightness of Stars E. Magnitude and Intensity II. The Sky and Its Motion A. The Celestial Sphere B. Precession III. The Cycles of the Sun A. The Annual Motion of the Sun B. The Seasons C. The Motion of the Planets Outline

V. Astronomical Influences on Earth's Climate A. The Hypothesis B. The Evidence Outline (continued)

Constellations In ancient times, constellations only referred to the brightest stars that appeared to form groups.

Constellations (2) They were believed to represent great heroes and mythological figures. Their position in the sky seemed to tell stories that were handed down from generation to generation over thousands of years.

Constellations (3) Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

Constellations (4) The stars of a constellation only appear to be close to one another. Usually, this is only a projection effect: The stars of a constellation may be located at very different distances from us.

Constellations (5) Stars are named by a Greek letter (  ) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Orion Betelgeuse Rigel Rigel =  Orionis Betelgeuse =  Orionis

Constellations (6) Some examples of easily recognizable constellations and their brightest stars

The Magnitude Scale First introduced by Hipparchus ( B.C.): Brightest stars: ~1 st magnitude Faintest stars (unaided eye): 6 th magnitude More quantitative: 1 st mag. stars appear 100 times brighter than 6 th mag. stars 1 mag. difference gives a factor of in apparent brightness (larger magnitude => fainter object!)

Betelgeuse Rigel Magnitude = 0.41 mag Magnitude = 0.14 mag The Magnitude Scale (Example) For a magnitude difference of 0.41 – 0.14 = 0.27, we find an intensity ratio of (2.512) 0.27 = In other words, Rigel is 1.28 times brighter than Betelgeuse.

The Magnitude Scale (2) Sirius (brightest star in the night sky): m v = Full moon: m v = Sun: m v = The magnitude scale system can be extended towards negative numbers (very bright) and numbers greater than 6 (faint objects):

The Celestial Sphere Celestial equator = projection of Earth’s equator onto the c.s. Zenith = Point on the celestial sphere directly overhead Nadir = Point on the c.s. directly underneath (not visible!) North celestial pole = projection of Earth’s north pole onto the c.s.

Distances on the Celestial Sphere The distance between two stars on the celestial sphere can only be given as the difference between the directions in which we see the stars. Therefore, distances on the celestial sphere are measured as angles, i.e., in degrees ( o ): Full circle = 360 o arc minutes (‘): 1 o = 60’ arc seconds (“): 1 ’ = 60”

The Celestial Sphere (2) From geographic latitude l (northern hemisphere), you see the celestial north pole l degrees above the northern horizon; l 90 o - l Celestial equator culminates 90º – l above the horizon. From geographic latitude – l (southern hemisphere), you see the celestial south pole l degrees above the southern horizon.

The Celestial Sphere (Example) The Celestial South Pole is not visible from the northern hemisphere. Horizon North Celestial North Pole South Celestial Equator Horizon New York City: l ≈ 40.7º

The Celestial Sphere (3)

Apparent Motion of The Celestial Sphere Looking north, you will see stars apparently circling counterclockwise around the Celestial North Pole.

Apparent Motion of The Celestial Sphere (2) Some constellations around the Celestial North Pole never set. These are called “circumpolar”. The circle on the celestial sphere containing the circumpolar constellations is called the “circumpolar circle”.

Apparent Motion of The Celestial Sphere (3) Looking east, you see stars rising and moving to the upper right (south) Looking south, you see stars moving to the right (west)

Precession (1) The Sun’s gravity is doing the same to Earth. The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession. At left, gravity is pulling on a slanted top. => Wobbling around the vertical.

Precession (2) As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. It will be closest to Polaris ~ A.D There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) ~ 12,000 years from now, the celestial north pole will be close to Vega in the constellation Lyra.

The Sun and Its Motions Earth’s rotation is causing the day/night cycle.

The Sun and Its Motions (2) The Sun’s apparent path on the sky is called the Ecliptic. Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere. Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations.

The Seasons Earth’s axis of rotation is inclined vs. the normal to its orbital plane by 23.5°, which causes the seasons.

The Seasons (2) The Seasons are caused only by a varying angle of incidence of the sun’s rays. We receive more energy from the sun when it is shining onto the Earth’s surface under a steeper angle of incidence.

The Seasons (3) The seasons are not related to Earth’s distance from the sun. In fact, Earth is slightly closer to the sun in (northern-hemisphere) winter than in summer. Light from the sun Steep incidence → Summer Shallow incidence → Winter

The Seasons (4) Northern summer = southern winter Northern winter = southern summer

The Seasons (5) Earth’s distance from the sun has only a very minor influence on seasonal temperature variations. Sun Earth in July Earth in January Earth’s orbit (eccentricity greatly exaggerated)

The Motion of the Planets The planets are orbiting the sun almost exactly in the plane of the Ecliptic. Jupiter Mars Earth Venus Mercury Saturn The Moon is orbiting Earth in almost the same plane (Ecliptic).

The Motion of the Planets (3) Mercury appears at most ~28° from the sun. It can occasionally be seen shortly after sunset in the west or before sunrise in the east. Venus appears at most ~46° from the sun. It can occasionally be seen for at most a few hours after sunset in the west or before sunrise in the east.

Astronomical Influences on Earth’s Climate Factors affecting Earth’s climate: Eccentricity of Earth’s orbit around the Sun (varies over period of ~ 100,000 years) Precession (Period of ~ 26,000 years) Inclination of Earth’s axis versus orbital plane Milankovitch Hypothesis: Changes in all three of these aspects are responsible for long-term global climate changes (ice ages).

Astronomical Influences on Earth’s Climate (2) Last glaciation End of last glaciation Polar regions receive more than average energy from the sun Polar regions receive less than average energy from the sun